Viscous oil, such as heavy oil or bitumen, residing in reservoirs that are sufficiently close to the surface may be mined.
Viscous oil residing in reservoirs that are too deep for commercial mining may be recovered by in situ processes. Commonly, viscous oil is produced from subterranean reservoirs using in situ recovery processes that reduce the viscosity of the oil enabling it to flow to the wells; otherwise, an economic production rate would not be possible. In commercial in situ viscous oil recovery processes, the temperature or pressure is modified or a solvent is added to reduce the viscosity or otherwise enhance the flow of the viscous oil within the reservoir. Such a solvent is referred to herein as a “viscosity reducing solvent” or simply a “solvent”.
Various in situ processes for recovering viscous oil are known including CSS (Cyclic Steam Stimulation), CSD (Constant Steam Drainage), SAGD (Steam Assisted Gravity Drainage), SA-SAGD (Solvent Assisted-Steam Assisted Gravity Drainage), VAPEX (Vapor Extraction), CSDRP (Cyclic Solvent-Dominated Recovery Process), LASER (Liquid Addition to Steam for Enhancing Recovery), SAVEX (Combined Steam and Vapor Extraction Process), water flooding, and steam flooding.
An example of CSS is described in U.S. Pat. No. 4,280,559 (Best). An example SAGD is described in U.S. Pat. No. 4,344,485 (Butler). An example of SA-SAGD is described in Canadian Patent No. 1,246,993 (Vogel). An example of VAPEX is described in U.S. Pat. No. 5,899,274 (Frauenfeld). An example of CSDRP is described in Canadian Patent No. 2,349,234 (Lim). An example of LASER is described in U.S. Pat. No. 6,708,759 (Leaute et al.). An example of SAVEX is described in U.S. Pat. No. 6,662,872 (Gutek).
In certain processes, such as in SAGD (Steam Assisted Gravity Drainage), a dedicated injection well and a dedicated production well are used.
In other processes, such as in CSS (Cyclic Steam Stimulation), the same well is used both for injecting a fluid and for producing oil. In CSS, cycles of steam injection, soak, and oil production are employed. Once the production rate falls to a given level, the well is put through another cycle of injection, soak, and production.
Certain viscous oil reservoirs are too deep to be commercially mined but lack an adequate top seal to employ in situ recovery methods using a fluid injectant (e.g. SAGD, SA-SAGD, and VAPEX) due to the potential loss of the injectant (for example steam or a solvent), or other gases, from the reservoir upwardly and, for instance, into an aquifer or to the surface. Significant deposits in the Athabasca oil sands region of Alberta, Canada, possess these characteristics.
Therefore, one limitation of solvent, steam, and solvent-steam combination in situ viscous oil recovery methods is the requirement for an adequate top seal. Reservoirs at this relatively shallow depth and lacking an adequate top seal are often silty sands or shales that may not adequately contain gases within the reservoir.
It would be desirable to have a method of recovering in situ viscous oil residing in reservoirs lacking an adequate top seal.